Control system for a work machine and method for controlling a hydraulic cylinder in a work machine

ABSTRACT

A control system for a work machine ( 101 ) including an electric machine ( 202 ), a hydraulic machine ( 204 ) and at least one hydraulic cylinder ( 108 ). The electric machine ( 202 ) is connected in a driving manner to the hydraulic machine ( 204 ). The hydraulic machine ( 204 ) is connected to a piston side ( 208 ) of the hydraulic cylinder ( 108 ) via a first line ( 210 ) and a piston-rod side ( 212 ) of the hydraulic cylinder ( 108 ) via a second line ( 214 ). The hydraulic machine ( 204 ) is adapted to be driven by the electric machine ( 202 ) and supply the hydraulic cylinder ( 108 ) with pressurized hydraulic fluid from a tank ( 216 ) in a first operating state and to be driven by a hydraulic fluid flow from the hydraulic cylinder ( 108 ) and drive the electric machine in a second operating state.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Application No.60/759,996 filed 18 Jan. 2006 and claims priority to Swedish ApplicationNo. 0600087-1 filed 16 Jan. 2006. Said applications are expresslyincorporated herein by reference in their entirety.

FIELD AND BACKGROUND

The present invention relates to a control system for a work machine anda method for controlling at least one hydraulic cylinder in a workmachine. Herein, the work machine is described in terms of a wheelloader. This is a preferred but is in no way limiting to the inventionas the he invention can also be used for other types of work machines(or work vehicles), such as an excavator loader (backhoe) and excavatingmachine.

The invention relates, for example, to controlling lifting and/ortilting cylinders for operating an implement.

More precisely, the invention relates to a control system whichcomprises a hydraulic machine that functions as both pump and motor. Thehydraulic machine is connected in a driving manner to an electricmachine which functions as both motor and generator.

The hydraulic machine therefore functions as a pump in a first operatingstate and supplies pressurized hydraulic fluid to the hydrauliccylinder. The hydraulic machine also functions as a hydraulic motor in asecond operating state and is driven by a hydraulic fluid flow from thehydraulic cylinder. The electric machine therefore functions as anelectric motor in the first operating state and as a generator in thesecond operating state.

The first operating state corresponds to a work operation, such aslifting or tilting, being carried out with the hydraulic cylinder.Hydraulic fluid is therefore directed to the hydraulic cylinder formovement of the piston of the cylinder. On the other hand, the secondoperating state is an energy recovery state.

SUMMARY

A first object of the invention is to provide a control system,preferably for a lifting and/or tilting function, which affords anopportunity for energy-efficient operation.

This object is achieved with a control system for a work machine, whichsystem comprises an electric machine, a hydraulic machine and at leastone hydraulic cylinder, the electric machine being connected in adriving manner to the hydraulic machine, the hydraulic machine beingconnected to a piston side of the hydraulic cylinder via a first lineand a piston-rod side of the hydraulic cylinder via a second line, thehydraulic machine being adapted to be driven by the electric machine andsupply the hydraulic cylinder with pressurized hydraulic fluid from atank in a first operating state and to be driven by a hydraulic fluidflow from the hydraulic cylinder and drive the electric machine in asecond operating state.

The hydraulic cylinder is preferably adapted to move an implement inorder to perform a work function. According to a first example, thehydraulic cylinder comprises a lifting cylinder for moving a loading armwhich is pivotably connected to a vehicle frame, the implement beingarranged on the loading arm. According to a second example, thehydraulic cylinder comprises a tilting cylinder for moving the implementwhich is pivotably connected to the loading arm.

The speed of the cylinder is preferably controlled directly by theelectric machine, that is to say no control valves are required betweenthe hydraulic machine and the cylinder for regulating direction andspeed of the movement. In some cases, on/off valves which open andrespectively close a communication for the hydraulic fluid flow arerequired.

A second object of the invention is to provide a method for controllinga hydraulic cylinder, preferably for a lifting and/or tilting function,which provides smooth operation and reduces jerking/jolting of thedriver.

This object is achieved by a method according to claim 31. It istherefore achieved with a method comprising the steps of detecting thata lowering movement of the implement is initiated, of before thelowering movement takes place pressurizing a first side of the piston ofthe hydraulic cylinder, which side is opposite a second side on whichsaid load acts, and then reducing the pressurization so that thelowering movement can start.

A third object of the invention is to provide a method which takesaccount of the size of the load and provides energy-efficient operation.

This object is achieved by a method according to claim 36. It istherefore achieved with a method comprising the steps of detecting aload acting on the implement, of comparing the size of the detected loadwith a predetermined load level, and of, if the detected load lies belowthe predetermined load level, bringing the piston-rod side of thehydraulic cylinder into flow communication with the piston side so thathydraulic fluid coming from the piston-rod side is brought to the pistonside without passing through the hydraulic machine.

A fourth object of the invention is to provide a method which providesenergy-efficient operation during movement of the implement.

This object is achieved by a method according to claim 38. It istherefore achieved with a method comprising the steps of delivering sucha pressure to the hydraulic cylinder that the implement is brought intoa basic position, of, in the event of a disturbance which results in adownward movement of the implement, allowing driving of a hydraulicmachine by a hydraulic fluid flow from the hydraulic cylinder, and ofregenerating the energy from the hydraulic machine in an electricmachine connected to it in a driving manner.

A fifth object of the invention is to provide a method which providesenergy-efficient springing of the movement of the implement duringtransport.

This object is achieved by a method according to claim 44. It istherefore achieved with a method comprising the steps of delivering sucha pressure to the hydraulic cylinder that the implement is brought intoa basic position, of bringing a first port of the hydraulic machine intoflow communication with a piston side of the hydraulic cylinder via afirst line and a second port of the hydraulic machine into flowcommunication with a piston-rod side of the hydraulic cylinder via asecond line, and of, in the event of a disturbance which results in anupward movement of the implement, supplying a corresponding quantity ofhydraulic fluid to the hydraulic cylinder and, in the event of adownward movement of the implement, draining a corresponding quantity ofhydraulic fluid from the hydraulic cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail below with referenceto the embodiments shown in the accompanying drawings, in which:

FIG. 1 shows a side view of a wheel loader;

FIGS. 2-6 show different embodiments of a control system for controllinga work function of the wheel loader;

FIG. 7 shows an embodiment of a control system for controlling a numberof functions of the wheel loader;

FIG. 8 shows a control system for controlling one or more of thefunctions of the wheel loader, and

FIG. 9 shows a further embodiment of the control system for controllinga work function of the wheel loader.

DETAILED DESCRIPTION

FIG. 1 shows a side view of a wheel loader 101. The wheel loader 101comprises a front vehicle part 102 and a rear vehicle part 103, whichparts each comprise a frame and a pair of drive axles 112, 113. The rearvehicle part 103 comprises a cab 114. The vehicle parts 102, 103 arecoupled together with one another in such a way that they can be pivotedin relation to one another about a vertical axis by means of twohydraulic cylinders 104, 105 which are connected to the two parts. Thehydraulic cylinders 104, 105 are thus arranged on different sides of acenter line in the longitudinal direction of the vehicle for steering,or turning the wheel loader 101.

The wheel loader 101 comprises an apparatus 111 for handling objects ormaterial. The apparatus 111 comprises a lifting arm unit 106 and animplement 107 in the form of a bucket which is mounted on the liftingarm unit. Here, the bucket 107 is filled with material 116. A first endof the lifting arm unit 106 is coupled rotatably to the front vehiclepart 102 for bringing about a lifting movement of the bucket. The bucket107 is coupled rotatably to a second end of the lifting arm unit 106 forbringing about a tilting movement of the bucket.

The lifting arm unit 106 can be raised and lowered in relation to thefront part 102 of the vehicle by means of two hydraulic cylinders 108,109, which are each coupled at one end to the front vehicle part 102 andat the other end to the lifting arm unit 106. The bucket 107 can betilted in relation to the lifting arm unit 106 by means of a thirdhydraulic cylinder 110, which is coupled at one end to the front vehiclepart 102 and at the other end to the bucket 107 via a link arm system.

A number of embodiments of a control system for the hydraulic functionsof the wheel loader 101 will be described in greater detail below. Theseembodiments relate to lifting and lowering of the lifting arm 106 viathe lifting cylinders 108, 109, see FIG. 1. However, the variousembodiments of the control system could also be used for tilting thebucket 107 via the tilting cylinder 110.

FIG. 2 shows a first embodiment of a control system 201 for performinglifting and lowering of the lifting arm 106, see FIG. 1. The hydrauliccylinder 108 in FIG. 2 therefore corresponds to the lifting cylinders108, 109 (although only one cylinder is shown in FIG. 2).

The control system 201 comprises an electric machine 202, a hydraulicmachine 204 and the lifting cylinder 108. The electric machine 202 isconnected in a mechanically driving manner to the hydraulic machine 204via an intermediate drive shaft 206. The hydraulic machine 204 isconnected to a piston side 208 of the hydraulic cylinder 108 via a firstline 210 and a piston-rod side 212 of the hydraulic cylinder 108 via asecond line 214.

The hydraulic machine 204 is adapted to function as a pump, be driven bythe electric machine 202 and supply the hydraulic cylinder 108 withpressurized hydraulic fluid from a tank 216 in a first operating stateand to function as a motor, be driven by a hydraulic fluid flow from thehydraulic cylinder 108 and drive the electric machine 202 in a secondoperating state.

The hydraulic machine 204 is adapted to control the speed of the piston218 of the hydraulic cylinder 108 in the first operating state. Nocontrol valves are therefore required between the hydraulic machine andthe hydraulic cylinder for said control. More precisely, the controlsystem 201 comprises a control unit 802, see FIG. 8, which iselectrically connected to the electric machine 202 in order to controlthe speed of the piston of the hydraulic cylinder 108 in the firstoperating state by controlling the electric machine.

The hydraulic machine 204 has a first port 220 which is connected to thepiston side 208 of the hydraulic cylinder via the first line 210 and asecond port 222 which is connected to the piston-rod side 212 of thehydraulic cylinder via the second line 214. The second port 222 of thehydraulic machine 204 is moreover connected to the tank 216 in order toallow the hydraulic machine, in the first operating state, to draw oilfrom the tank 216 via the second port 222 and supply the oil to thehydraulic cylinder 108 via the first port 220.

In certain situations, such as when it is desired to press a materialdown or to flatten something, it is necessary to lower the bucket 107with more force than is the case when only the load drives the movementof the piston 218. Such intensified lowering is usually referred to as“power down”. This power down function can also be used for lifting thevehicle. The control system 201 comprises a means 224 for controllingpressure, which pressure means 224 is arranged on a line 226 between thesecond port 222 of the hydraulic machine 204 and the tank 216 in orderto allow pressure build-up on the piston-rod side 212. More precisely,the pressure control means 224 comprises an electrically controlledpressure-limiting valve.

The control system 201 also comprises a sensor 228 for sensing pressureon the piston side 208 of the hydraulic cylinder 108. When a lowpressure value is detected on the piston side, the line 226 to the tankis blocked via the pressure-limiting valve 224, which results in thepressure in the line 214 to the piston-rod side being increased and saidintensified downward movement (power down) being obtained. Duringlowering, the pressure sensor registers that the pressure is below acertain level (for example 20 bar) on the piston side. The pressurelevel on the electrically controlled pressure limiter is then increasedto a suitable level so that pressure build-up takes place in thepiston-rod side.

The first port 220 of the hydraulic machine 204 is connected to the tank216 via a first suction line 230. A means 232, in the form of anon-return valve, is adapted to allow suction of hydraulic fluid fromthe tank and obstruction of a hydraulic fluid flow to the tank throughthe suction line 230.

The second port 222 of the hydraulic machine 204 is connected to thetank 216 via a second suction line 234. A means 236, in the form of anon-return valve, is adapted to allow suction of hydraulic fluid fromthe tank and obstruction of a hydraulic fluid flow to the tank throughthe suction line 234.

A means 237 for opening/closing is arranged on the second line 214between the second port 222 of the hydraulic machine 204 and thepiston-rod end 212 of the hydraulic cylinder 108. This means 237comprises an electrically controlled valve with two positions. In afirst position, the line 214 is open for flow in both directions. In asecond position, the valve has a non-return valve function and allowsflow in only the direction toward the hydraulic cylinder 108. Duringlifting movement, the electric valve 237 is opened and the rotationalspeed of the electric machine 202 determines the speed of the piston 218of the hydraulic cylinder 108. Hydraulic fluid is drawn from the tank216 via the second suction line 234 and is pumped to the piston side 208of the hydraulic cylinder 108 via the first line 210.

An additional line 242 connects the second port 222 of the hydraulicmachine 204 and the tank 216.

A means 243 for opening/closing is arranged on the first line 210between the first port 220 of the hydraulic machine 204 and the pistonend 208 of the hydraulic cylinder 108. This means 243 comprises anelectrically controlled valve with two positions. In a first position,the line 210 is open for flow in both directions. In a second position,the valve has a non-return valve function and allows flow in only thedirection toward the hydraulic cylinder 108.

According to an embodiment for lowering the implement, it is firstdetected that a lowering movement is initiated. The electric valve 243is closed. Before the lowering movement takes place, a first side 208 ofthe piston 218 of the hydraulic cylinder is pressurized, which side isopposite a second side on which said load acts. In other words, thepiston side 208 is pressurized. Before the lowering movement takesplace, the hydraulic machine 204 is driven in a first rotation directionso that said first side 208 of the piston of the hydraulic cylinder ispressurized. The hydraulic machine 204 is therefore rotated by a certainangle in the “wrong direction”. A sensor 248 is adapted to sense theposition of the piston rod. A detected upward movement of the piston rodindicates that the pressurization is complete. According to analternative, the pump 204 is rotated by a predetermined angle in the“wrong direction”.

The valve 243 is then opened to the piston side 208, the rotationdirection is changed for the hydraulic machine 204 and the loweringmovement starts. The electrically controlled pressure limiter may needto be adjusted slightly in order to improve refilling to the piston-rodside.

The hydraulic machine is therefore allowed to rotate in a secondrotation direction, opposite the first rotation direction, whereupon thelowering movement can start. The pressure applied is therefore reducedso that the lowering movement can start. A hydraulic flow from thehydraulic cylinder 108 drives the hydraulic machine 204 in the secondrotation direction. More precisely, the pressurization of the first side208 of the hydraulic cylinder is reduced gradually so that a smoothlowering movement is achieved.

Pressurization can also be effected by the electric machine 202 firstbeing driven with a certain torque in the “wrong direction”, where thetorque level is based on the value of the pressure sensor 228immediately before.

If the bucket 107 should stop suddenly during a lowering movement (whichcan happen if the bucket strikes the ground), the hydraulic machine 204does not have time to stop. In this state, hydraulic fluid can be drawnfrom the tank 216 via the suction line 230 and on through the additionalline 242.

The electrically controlled valves 237, 243 function as load-holdingvalves. They are closed in order that electricity is not consumed whenthere is a hanging load and also in order to prevent dropping when thedrive source is switched off According to an alternative, the valve 237on the piston-rod side 212 is omitted. However, it is advantageous toretain the valve 237 because external forces can lift the lifting arm106.

A filtering unit 238 and a heat exchanger 240 are arranged on theadditional line 242 between the second port 222 of the hydraulic machine204 and the tank 216. An additional filtering and heating flow can beobtained by virtue of the hydraulic machine 204 driving a circulationflow from the tank 216 first via the first suction line 230 and then viathe additional line 242 when the lifting function is in a neutralposition. Before the tank, the hydraulic fluid thus passes through theheat exchanger 240 and the filter unit 238.

There is another possibility for additional heating of the hydraulicfluid by pressurizing the electrically controlled pressure limiter 224at the same time as pumping-round takes place to the tank in the waymentioned above. This can of course take place when the lifting functionis used.

The electrically controlled pressure limiter 224 can also be used as aback-up valve for refilling to the piston-rod side 212 when loweringtakes place. The counter-pressure can be varied as required and kept aslow as possible, which saves energy. The counter-pressure can be lowerthe hotter the oil is and lower the lower the lowering speed is. Whenthe filtering flow is run, the counter-pressure can be zero.

A first pressure-limiting valve 245 is arranged on a line which connectsthe first port 220 of the hydraulic machine 204 to the tank 216. Asecond pressure-limiting valve 247 is arranged on a line which connectsthe piston side 208 of the hydraulic cylinder 108 to the tank 216. Thetwo pressure-limiting valves 245, 247 are connected to the first line210 between the hydraulic machine 204 and the piston side 208 of thehydraulic cylinder 108 on different sides of the valve 243. The twopressure-limiting valves 245, 247, which are also referred to as shockvalves, are spring-loaded and adjusted to be opened at differentpressures. According to an example, the first pressure-limiting valve245 is adjusted to be opened at 270 bar, and the secondpressure-limiting valve 247 is adjusted to be opened at 380 bar.

When the work machine 101 is driven toward a heap of gravel or stonesand/or when the implement is lifted/lowered/tilted, the movement of thebucket may be counteracted by an obstacle. The pressure-limiting valves245, 247 then ensure that the pressure is not built up to levels whichare harmful for the system.

According to a first example, the bucket 107 is in a neutral position,that is to say stationary in relation to the frame of the front vehiclepart 102. When the wheel loader 101 is driven toward a heap of stones,the second pressure limiter 247 is opened at a pressure of 380 bar.

During ongoing lowering, the valve 243 on the first line 210 between thehydraulic machine 204 and the piston side 208 of the hydraulic cylinder108 is open. When the lifting arm 106 is lowered, the first pressurelimiter 245 is opened at a pressure of 270 bar. If an external forceshould force the loading arm 106 upward during a lowering operation withpower down, the pressure limiter 224 on the line 226 between the secondport 222 of the hydraulic machine 204 and the tank 216 is opened.

According to an alternative to the pressure-limiting valves 245, 247being adjusted to be opened at a predetermined pressure, thepressure-limiting valves can be designed with variable opening pressure.According to a variant, the pressure-limiting valves 245, 247 areelectrically controlled. If electric control is used, only one valve 247is sufficient for the shock function. This valve 247 is controlleddepending on whether the valve 243 is open or closed. The openingpressure can be adjusted depending on activated or non-activatedlifting/lowering function and also depending on the cylinder position.

A method for regenerating energy when the implement 107 is moved duringmovement of the work machine 101 is described below with reference toFIG. 2. The method can be said to constitute an active springing systemfor the lifting function. The method can either be selected by anoperator via a control element or a control, such as a knob or lever, inthe cab or be initiated automatically.

A sensor 248 is adapted for sensing the position of the lifting arm 106in relation to the frame of the front vehicle part 102. Here, the sensor248 is adapted to detect the position of the piston rod. The sensor 248could alternatively detect the angular position of the loading arm 106relative to the frame. The sensor 248 detects the position of theimplement repeatedly, essentially continuously, and producescorresponding signals.

A control unit 802 (see FIG. 8) receives the position signals from thesensor 248. The control unit 802 is usually referred to as a CPU(central processing unit) and comprises a microprocessor and a memory.

The position of the loading arm 106 is stored in the memory before theenergy regeneration function is activated. When the function isactivated, the two valves 237 and 243 on both sides of the liftingcylinder 108 are opened. The hydraulic machine 204 is controlled so thatsuch a pressure is delivered to the hydraulic cylinder 108 that theimplement 107 is brought into a basic position. The loading arm 106 istherefore held in position with a certain torque.

During movement of the wheel loader 101, that is to say transport, theloading arm 106 will be acted on by vertical forces owing to the weightof the load and irregularities of the ground and move up and down. Thesensor 248 registers such disturbances which result in the loading arm106 being moved from the basic position.

In the event of a disturbance which results in a downward movement ofthe implement 107, the control unit 802 produces a signal for theelectric machine 202 which allows the hydraulic machine 204 to be drivenby a hydraulic fluid flow from the hydraulic cylinder 108 and the energyfrom the hydraulic machine 204 is regenerated in the electric machine202. More precisely, the first port 220 of the hydraulic machine 204 isbrought into flow communication with the piston side 208 of thehydraulic cylinder 108. The control unit 802 therefore sends a signal tothe valve 243 on the first line 210, which is thus opened. When thelifting arm 106 moves downward, the basic position is passed through,the counter-torque of the electric machine 204 increasing so that themovement of the lifting arm is braked and in the end stops. Oil is thenpumped into the cylinder 108 so the lifting arm 106 moves upward again.

If a disturbance means that the lifting arm 106 moves upward, thecontrol unit 802 registers this. The control unit controls the hydraulicmachine 204 (via the electric machine 202) so that the hydraulic machinefollows with a certain torque and fills hydraulic fluid to the pistonside 208. The torque applied decreases depending on how far from thebasic position the lifting arm 106 is. A springing function is thusobtained.

More precisely, a second port 222 of the hydraulic machine 204 isbrought into flow communication with the piston-rod side 212 of thehydraulic cylinder 108.

The hydraulic cylinder 108 is controlled continuously so that theimplement 107 is kept within a predetermined range around the basicposition. Adjustment is also carried out continuously between thedisturbances so that the loading arm 106 does not move too far from thebasic position.

If there are few disturbances, the valve 243 on the piston side 208 canbe closed temporarily in order to save the energy which is consumed forholding the load.

The function also damps shocks which occur as a result of externalforces such as, for example, collision with the bucket 107.

According to a development of the energy regeneration function, pressuresensors are used for registering the course of the pressure variationswhich occur in the event of a disturbance. If pressure sensors are used,the valve 243 on the piston side 208 can if appropriate be closed aslong as no lowering movement takes place (depending on how quickly it ispossible to open in the event of a disturbance).

The hydraulic machine 204 is controlled so that a springing function isachieved. In other words, if a disturbance presses the lifting arm 106down, the hydraulic machine 204 regenerates electricity and at the sametime the torque is increased so that braking of the movement takes place(like a spring). This spring characteristic can be dependent on a numberof different parameters and have a different appearance.

According to a preferred embodiment, the spring characteristic isdependent on the following parameters:

1) Level of Disturbance Force

The same spring travel is obtained for the same disturbance force(irrespective of the weight of the load). The spring travel is longerthe greater the disturbance force is. The disturbance force can beregistered via pressure sensors or the derivative on the positionsensor.

2) How Heavy the Load is

It is possible, for example, to measure the pressure in the liftingcylinder and if appropriate in the tilting cylinder. According to afirst variant, the springing is controlled so that the heavier thedetected load is the shorter the spring travel. According to a secondvariant, the springing is controlled so that the lighter the detectedload is the shorter the spring travel.

3) Type of Handling

The computer registers the type of handling (bucket, pallet fork, timberfork etc.) in a manner known per se.

4) Type of Handling Mode

Different characteristics if the machine is in transport mode or if workis in progress with the function. This could be indicated, for example,via machine speed and/or whether lever movement takes place.

The damping in the system is determined by the size of the torque whichthe pump applies when the unit is to be raised again after being presseddown. This torque application (spring characteristic) can also be afunction of the parameters above.

FIG. 3 shows a second embodiment of the control system 301. Here, thefirst port 220 of the hydraulic machine 204 is connected to thepiston-rod side 212 of the hydraulic cylinder 108 via a line 302 whichconnects the piston-rod side 212 and the piston side 208 of thehydraulic cylinder 108 in parallel to the hydraulic machine 204. A means304 for flow control, in the form of an electrically controlled on/offvalve, is arranged on said parallel line 302 in order to control theflow communication between the piston-rod side 212 and the piston side208. By virtue of the valve 304, the maximum flow via the hydraulicmachine 204 can be lowered, that is to say the pump displacement can bereduced or a lower maximum speed can be used.

The pressure sensor 228 indicates whether the weight of the load isbelow or above a predetermined value, which indicates whether the loadis considered to be light or heavy. In the case of a lifting movement ofa light load, the additional valve 304 is opened, which results in morerapid lifting being possible by virtue of hydraulic fluid for the pistonside 208 being obtained both from the hydraulic machine 204 and from thepiston-rod side 212. The electric valve 237 on the second line 214 onthe piston-rod side 212 is therefore closed.

In the case of a lifting movement of a heavy load, the electric valve237 on the second line 214 on the piston-rod side 212 is opened. Theelectric valve 304 on the parallel line 302 is closed. The lifting takesplace somewhat more slowly due to the fact that the whole piston side208 has to be filled by the hydraulic machine 204.

In the case of a light load, lowering can take place more rapidly, dueto the fact that only the volume of the piston rod passes via thehydraulic machine 204. First, the additional valve 304 on the parallelline 302 is opened. Before the lowering movement, pressurization takesplace, for example by the electric machine 202 first being driven with acertain torque in the “wrong direction”, where the torque level is basedon the value of the pressure sensor 228 immediately before.Alternatively, the hydraulic machine 204 rotates by a certain angle inthe “wrong direction”. The valve 243 on the first line 210 is thenopened to the piston side 208, the rotation direction of the hydraulicmachine 204 is changed, and the lowering movement starts.

The lowering movement of a heavy load can be performed as follows: thepressure sensor 228 indicates heavy load. The additional valve 304 onthe parallel line 302 is closed. In this state, all the flow from thepiston side 208 passes via the hydraulic machine 204. The electricallycontrolled pressure limiter may need to be adjusted slightly in order toimprove refilling to the piston-rod side 212.

According to a preferred embodiment, the pressure sensor 228 thereforedetects a load acting on the implement and generates a correspondingsignal. The control unit 802, see FIG. 8, compares the size of thedetected load with a predetermined load level. If the detected load isbelow the predetermined load level, a corresponding signal is sent tothe valve 304, which is opened, the piston-rod side 212 of the hydrauliccylinder 108 being brought into flow communication with the piston side208 so that hydraulic fluid coming from the piston-rod side is broughtto the piston side without passing through the hydraulic machine 204. Ifon the other hand the detected load is above the predetermined loadlevel, a corresponding signal is sent to the valve 237, which is opened,the piston-rod side of the hydraulic cylinder being brought into flowcommunication with the second port 222 of the hydraulic machine 204 sothat hydraulic fluid coming from the piston-rod side 212 is brought tothe second port of the hydraulic machine.

FIG. 4 shows a third embodiment of the control system 401. A flowcontrol means 402, in the form of an electrically controlledproportional valve, is connected on a line 404 which extends between thefirst line 210 and the tank 216 in order to allow a certain leakage flowfrom the hydraulic machine 204 to the tank at the start of a liftingmovement. The hydraulic machine 204 thus has a certain basic revolutionbefore lifting takes place. This reduces starting friction. The valve402 can then be closed gradually the greater the lifting speed becomes.The valve 402 is a small valve which only produces an adequate drainageflow so that the hydraulic machine 204 starts working before thecylinder movement starts.

A flow control means 406, in the form of an electrically controlledproportional valve, is connected on the first line 210 between thehydraulic machine 204 and the piston side 208 of the hydraulic cylinderin order to control the size of the hydraulic fluid flow from thehydraulic cylinder 108 to the hydraulic machine 204 at the start of thelowering movement. At the start of the lowering movement, the electricmachine 202 has a low counter-torque in order to prevent startingfriction and a jerky start. The valve 406 is opened proportionally andthe piston speed is controlled. In parallel with the valve 406 beingopened, the counter-torque in the electric machine 202 is increased andthe hydraulic machine 204 gradually takes over the speed control of thelowering movement. In the end, the valve 406 is fully open and thelowering speed is controlled completely by the electric machine 202.

FIG. 5 shows a fourth embodiment of the control system 501. Thehydraulic machine 204 can be connected via a connection means 502 to anadditional hydraulic actuator 504 which is adapted to perform a workfunction which is separate from a work function performed by saidhydraulic cylinder 108. Here, the connection means 502 consists of anelectrically controlled directional valve. The additional work functioncan be, for example, implement locking or an emergency pump for thesteering function.

FIG. 6 shows a fifth embodiment of the control system 601, which is adevelopment of the first embodiment, see FIG. 2. Here, said means forallowing suction of hydraulic fluid from the tank 216 through thesuction lines 230, 234 consist of electrically controlled on/off valves632, 636 instead of non-return valves. This reduces problems ofcavitation on the suction side.

The valve 636 which connects the second port 222 of the hydraulicmachine 204 to the tank 216 can be open when the hydraulic machinerotates in the direction so that hydraulic fluid passes to the cylinder108. The valve 636 is closed when the rotation is changed.

The valve 632 which connects the first port 220 of the hydraulic machine204 to the tank 216 is opened when the filtering and heating flow isrun. The valve 636 may also need to be opened if the unit stops deadduring ongoing lowering, which results in cavitation occurring onaccount of the fact that the hydraulic machine 202 does not have time tostop. Such a course of events can be registered by, for example,registering the state of the hydraulic machine 202 and the state of thecylinder 108.

FIG. 7 shows a control system 701 comprising a subsystem 707 for thelifting function, a subsystem 709 for the tilting function, a subsystem711 for the steering function and a subsystem 731 for an additionalfunction. A number of different system embodiments for the liftingfunction have been described above.

The subsystem 709 shown in FIG. 7 for the tilting function has aconstruction corresponding to the system for the lifting function. FIG.7 illustrates the electric machine with reference sign 703 and thehydraulic machine with reference sign 705. For the tilting function, apressure-limiting valve 702, or shock valve, is added, which connectsthe piston-rod side of the tilting cylinder 110 to the tank.

The subsystem 711 shown in FIG. 7 for the steering function comprisessaid first and second steering cylinders 104, 105, which are adapted forframe-steering the wheel loader 101. The system also comprises a firstdrive unit 704 and a second drive unit 706, which each comprise anelectric machine 708, 710 and a hydraulic machine 712, 714. Eachelectric machine 708, 710 is connected in a driving manner to itsassociated hydraulic machine 712, 714.

A first 712 of the two hydraulic machines is connected to a piston side716 of the first hydraulic cylinder 104 and a piston-rod side 718 of thesecond hydraulic cylinder 105. A second 714 of the two hydraulicmachines is connected to a piston side 720 of the second hydrauliccylinder 105 and a piston-rod side 722 of the first hydraulic cylinder104.

For steering the wheel loader 101 in a direction (for example to theright), a first of the hydraulic machines 712 is adapted to be driven byits associated electric machine 708 and to supply the hydrauliccylinders 104, 105 with pressurized hydraulic fluid from the tank 216,and the second hydraulic machine 714 is adapted to be driven by ahydraulic fluid flow from the hydraulic cylinders 104, 105 and to driveits associated electric machine 710, and vice versa.

The hydraulic machines are therefore driven in opposite directionsduring operation.

A first electrically controlled control means (control valve) 724 isarranged between the hydraulic machine 712 of the first drive unit 704and the steering cylinders 104, 105, and a second electricallycontrolled control means (control valve) 726 is arranged between thehydraulic machine 714 of the second drive unit 706 and the steeringcylinders 104, 105.

The subsystem 731 shown in FIG. 7 for the additional function preferablycomprises only one drive unit 734 for providing all the additionalfunctions. This means that it is easier to add an additional function,see arrow 766, as only a valve unit has to be added. The drive unit 734comprises a pump 736 which is driven mechanically by an electric motor738. This additional function can consist of, for example, the implement107 comprising parts which are movable relative to one another, themovement of which is controlled. Such functions can consist of asweeping roller, clamping arms etc.

A hydraulic actuator in the form of a hydraulic cylinder 732 is adaptedfor carrying out the movement in the control system 731 shown. The pump736 is connected to a piston side 740 and a piston-rod side 742 via afirst and a second line 744, 746. An inlet valve in the form of anelectrically controlled proportional valve 748, 750 is arranged on eachof the first and second lines 744, 746. The piston side 740 and thepiston-rod side 742 are connected to the tank 216 via a third and fourthline 752, 754. An outlet valve in the form of an electrically controlledproportional valve 756, 758 is arranged on each of the third and fourthlines 752, 754. A pressure sensor 760, 762 is arranged on each of thethird and fourth lines 752, 754. An additional pressure sensor 764 isarranged on the line downstream of the pump 736 and upstream of theinlet valves 748, 750.

According to an alternative, more pumps and if appropriate electricmotors can be added for the purpose of increasing the maximum flow. Thepump for the lifting or the tilting function can moreover be connectedin parallel for any topping of the flow. Functions with another type ofvalve can also be added.

The additional function can be controlled via inlet control: onactivation of a function, the load pressure in the cylinder 732 isregistered. The pump 736 is set with a torque which gives a certainlevel of higher pressure before the inlet valve 748, 750, which isregistered via the pressure sensor 764 before the valve. This means thatthe inlet valve 748, 750 has a known pressure drop. By virtue of thefact that the pressure drop can be read off, the flow can now beadjusted via control of the inlet valve (regulating the opening area).If a number of functions are running at the same time, the pump 736builds up a torque which is a certain level higher than the highestregistered load pressure. The outlet valve 756, 758 opens to a levelwhich gives a specific counter-pressure, which can be read off via thepressure sensor 760, 762 on the outlet side of the cylinder 732. If thecounter-pressure is higher on account of a hanging load, the outletvalve 756, 758 is regulated so that the pressure on the inlet side doesnot fall below a certain level. Functions which have a motor instead ofa cylinder can be regulated in the same way.

The additional function can alternatively be controlled via outletcontrol: the pump 736 is set with a torque which gives a certainpressure level before the outlet valve 756, 758, which is registered viathe pressure sensor 760, 762 before the outlet valve. This means thatthe outlet valve 756, 758 has a pressure drop which is known (the tankside is in principle pressureless). According to analternative/supplement, a pressure sensor is arranged on the tank side.It is then possible to have control of the pressure drop across thevalve (in some cases the system is not pressureless).

By virtue of the fact that the pressure drop can be read off, the flowcan now be adjusted via control of the outlet valve 756, 758 (regulatingthe opening area). If a number of functions are running at the sametime, the pump builds up a torque which gives a certain level ofpressure at the pressure sensor (on the outlet side) which has thelowest pressure.

The inlet valve 748, 750 can be opened fully so that no pressure dropoccurs (lower losses). If it is hanging load, the cylinder 732 drives,or if a pump flow deficiency occurs, the outlet valve 756, 758 is alsoregulated so that the pressure on the inlet side of the cylinder 732does not fall below a certain level. Prioritizing/weighting can takeplace between the functions is the pump flow is not sufficient.

Functions which have a motor instead of a cylinder can be regulated inthe same way.

If use is made of a function which has a hydraulic motor (for example asweeping roller), both the inlet valve 748, 750 and the outlet valve756, 758 can be opened fully so that no pressure drop is generated. Thespeed of the sweeping roller is then controlled directly via the speedof the pump 736. If another function is temporarily controlledsimultaneously, it is possible to change over temporarily to inletcontrol or outlet control.

The control system 731 affords opportunities for a maximum feed pressurelimitation. The pressure can be read off via the pressure sensor, andthe inlet valve can be throttled when the pressure becomes too high.

The control system 731 also affords opportunities for dealing with ashock pressure. The pressure can be read off via pressure sensor, andthe outlet valve can drain to the tank when the pressure level becomestoo high.

According to a development, a back-up valve can be added after the valve756, 758 on the outlet side (toward the tank 216), together withrefilling valves for the cylinder 732. This provides more available pumpflow when a number of functions are running simultaneously and then if afunction has a load which drives the flow.

FIG. 8 shows a control system for controlling the control system 701shown in FIG. 7 for the lifting function, the tilting function, thesteering function and the additional function. A number of elements, orcontrols, 804, 806, 808, 810, 812, 814 are arranged in the cab 114 formanual operation by the driver and are electrically connected to thecontrol unit 802 for controlling the various functions. A wheel 804 anda control lever 806 are adapted for controlling the steering function. Alifting lever 808 is adapted for the lifting function and a tiltinglever 810 is adapted for the tilting function. A lever 812 is adaptedfor controlling the third function, and an additional control 814 isadapted for pump control (adjustable flow) for the third function. Anumber of additional functions with associated controls can be added.

The electric machines 202, 703, 708, 710, 738 are electrically connectedto the control unit 802 in such a way that they are controlled by thecontrol unit and that they can provide operating state signals to thecontrol unit.

The control system comprises one or more energy storage means 820connected to one or more of said electric machines 202, 703, 708, 710,738. The energy storage means 820 can consist of a battery or asupercapacitor, for example. The energy storage means 820 is adapted toprovide the electric machine with energy when the electric machine 202is to function as a motor and drive its associated pump 204. Theelectric machine 202 is adapted to charge the energy storage means 820with energy when the electric machine 202 is driven by its associatedpump 204 and functions as a generator.

The wheel loader 101 also comprises a power source 822 in the form of aninternal combustion engine, which usually consists of a diesel engine,for propulsion of the vehicle. The diesel engine 822 is connected in adriving manner to the wheels of the vehicle via a drive line (notshown). The diesel engine 822 is moreover connected to the energystorage means 820 via a generator (not shown) for energy transmission.

It is possible to imagine alternative machines/units adapted forgenerating electric power. According to a first alternative, use is madeof a fuel cell which provides the electric machine with energy.According to a second alternative, use is made of a gas turbine with anelectric generator for providing the electric machine with energy.

FIG. 8 also shows the other components which are connected to thecontrol unit 802 according to the first embodiment of the control systemfor the lifting function, see FIG. 2, such as the electricallycontrolled valves 224, 237, 243, the position sensor 248 and thepressure sensor 228. It will be understood that corresponding componentsfor the tilting function and the steering function and the additionalfunction are connected to the control unit 802.

FIG. 9 shows a further embodiment of the control system 901. The controlsystem 901 comprises a hydraulic cylinder 902 which is reversed, whichmeans that a load 904 draws the cylinder out via its weight. Thiscontrol system 901 can be said to be a variant of the control system 201according to the first embodiment, see FIG. 2.

In order to bring about necessary refilling to the piston side 906 ofthe cylinder 902 during a lowering movement, the system comprises anadditional, smaller pump 908. The smaller pump has a driving connectionto the hydraulic machine 204.

During lowering, the hydraulic fluid passes from the piston-rod side 910of the cylinder 902 to the piston side 906 via the larger hydraulicmachine 204. The small pump 908 contributes to pumping hydraulic fluidfrom the tank 216 to the piston side 906 via a suction line 912. Duringa lifting movement, the small pump 908 performs no useful work. Thesmall pump 908 only pumps hydraulic fluid round through itself via asmall non-return valve 914. The non-return valve 914 is thereforeconnected between an inlet side 916 and an outlet side 918 of theadditional pump 908, so that, during a lifting movement, the pump 908only pumps hydraulic fluid in a circuit 920 comprising the non-returnvalve 914. The non-return valve 914 is therefore arranged in parallelwith the small pump 908.

Otherwise, this system 901 functions similarly to the basic system (seeFIG. 2), apart from the filtering and heating flow being a littlegreater.

According to a previously known pump, there is a regulator in the pump,which provides a pressure-limiting function so that the displacement ofthe pump is adjusted down in the event of pressure being too high.According to one embodiment of the control method, the built-inpressure-limiting function of the pump can be omitted, and asimpler/cheaper pump can therefore be used as a hydraulic machine.

A first embodiment of the regulating method comprises the steps ofdetecting an operating parameter and of generating a correspondingparameter signal, of determining a level of said pressure based on thelevel of the detected operating parameter, of comparing the determinedpressure level with a predetermined maximum level and of controlling thehydraulic machine so that a delivered pressure lies below thepredetermined maximum level. More precisely, the parameter signalgenerated is received by the control unit (computer) and is processed,after which a control signal is sent to the electric machine which has adriving connection to the hydraulic machine to reduce the deliveredtorque if the determined pressure level exceeds the predeterminedmaximum level.

The preferred embodiment comprises the step of detecting a torquedelivered by the electric machine and of determining the level of saidpressure based on the detected torque. Furthermore, a level of saidpressure based on at least the detected torque and the displacement ofthe hydraulic machine is calculated.

According to an alternative to detecting the delivered torque of theelectric machine, it is possible to detect the pressure of the hydraulicfluid in a line downstream of the hydraulic machine and to compare thedetected pressure level with the predetermined maximum level.

The invention is not to be regarded as being limited to the illustrativeembodiments described above, but a number of further variants andmodifications are conceivable within the scope of the following patentclaims.

1. A control system for a work machine (101) comprising: an electricmachine (202); an hydraulic machine (204); and at least one hydrauliccylinder (108); the electric machine (202) being connected in a drivingmanner to the hydraulic machine (204); the hydraulic machine (204) beingconnected to a piston side (208) of the hydraulic cylinder (108) via afirst line (210) and a piston-rod side (212) of the hydraulic cylinder(108) via a second line (214); and the hydraulic machine (204) beingadapted to be driven by the electric machine (202) and supply thehydraulic cylinder (108) with pressurized hydraulic fluid from a tank(216) in a first operating state and to be driven by a hydraulic fluidflow from the hydraulic cylinder (108) and drive the electric machine ina second operating state.
 2. The control system as recited in claim 1,wherein the hydraulic machine (204) is adapted to control the speed ofthe piston (218) of the hydraulic cylinder (108) in the first operatingstate.
 3. The control system as recited in claim 1, wherein the controlsystem comprises a control unit (802) which is electrically connected tothe electric machine (202) in order to control the speed of the piston(218) of the hydraulic cylinder (108) in the first operating state bycontrolling the electric machine.
 4. The control system as recited inclaim 1, wherein the hydraulic machine (204) has a first port (220)which is connected to the piston side (208) of the hydraulic cylinder(108) via the first line (210) and a second port (222) which isconnected to the piston-rod side (212) of the hydraulic cylinder (108)via the second line (214).
 5. The control system as recited in claim 1,wherein a second port (222) of the hydraulic machine (204) is connectedto the tank (216) in order to allow the hydraulic machine (204), in thefirst operating state, to draw oil from the tank via the second port(222) and supply the oil to the hydraulic cylinder via a first port(220).
 6. The control system as recited in claim 4, wherein the systemcomprises a means (224) for controlling pressure, which pressure means(224) is arranged on a line (226) between the second port (222) of thehydraulic machine and the tank in order to allow pressure build-up onthe piston-rod side (212).
 7. The control system as claimed in claim 6,wherein the pressure control means (224) comprises an electricallycontrolled pressure-limiting valve.
 8. The control system as recited inclaim 1, wherein the system comprises a sensor (228) for sensingpressure on the piston side (208) of the hydraulic cylinder.
 9. Thecontrol system as recited in claim 1, wherein a first port (220) of thehydraulic machine is connected to the tank (216) via a suction line(230).
 10. The control system as recited in claim 9, wherein a means(232, 632) is arranged on the suction line (230) in order to allowsuction of hydraulic fluid from the tank and obstruction of a hydraulicfluid flow to the tank.
 11. The control system as recited in claim 10,wherein the means comprises a non-return valve (232).
 12. The controlsystem as recited in claim 10, wherein the means comprises anelectrically controlled on/off valve (632).
 13. The control system asrecited in claim 1, wherein a second port (222) of the hydraulic machineis connected to the tank (216) via a suction line (234).
 14. The controlsystem as recited in claim 13, wherein a means (236, 636) is arranged onthe suction line (234) in order to allow suction of hydraulic fluid fromthe tank and obstruction of a hydraulic fluid flow to the tank.
 15. Thecontrol system as recited in claim 14, wherein the means comprises anon-return valve (236).
 16. The control system as recited in claim 14,wherein the means comprises an electrically controlled on/off valve(636).
 17. The control system as recited in claim 1, wherein a secondport (222) of the hydraulic machine (204) is connected to the tank (216)via a line (242).
 18. The control system as recited in claim 17, whereina filtering unit (238) is arranged on the line (242) between the secondport (222) of the hydraulic machine (204) and the tank (216).
 19. Thecontrol system as recited in claim 1, wherein the hydraulic machine(204) can be connected via a connection means (502) to a hydraulicactuator (504) which is adapted to perform a work function which isseparate from a work function performed by said hydraulic cylinder(108).
 20. The control system as recited in claim 4, wherein a firstport (220) of the hydraulic machine (204) is connected to the piston-rodside (212) of the hydraulic cylinder (108).
 21. The control system asrecited in claim 1, wherein the system comprises a line (302) whichconnects the piston-rod side (212) and the piston side (208) of thehydraulic cylinder (108) in parallel to the hydraulic machine (204). 22.The control system as recited in claim 21, wherein the system comprisesa means (304) for flow control, which is arranged on said parallel line(302) in order to control the flow communication between the piston-rodside (212) and the piston side (208).
 23. The control system as recitedin claim 1, wherein a first port (220) of the hydraulic machine (204) isconnected to a piston side (208) of the hydraulic cylinder (108) via afirst line (210), and wherein a flow control means (402) is connectedbetween the first line (210) and the tank (216) in order to allow acertain leakage flow from the hydraulic machine (204) to the tank at thestart of a lifting movement.
 24. The control system as recited in claim1, wherein a first port (220) of the hydraulic machine (204) isconnected to a piston side (208) of the hydraulic cylinder (108) via afirst line (210), and wherein a flow control means (406) is connected onthe first line (210) in order to control the size of the hydraulic fluidflow from the hydraulic cylinder (108) to the hydraulic machine (204) atthe start of a lowering movement.
 25. The control system as recited inclaim 1, wherein the hydraulic cylinder is adapted to move an implement(107) in order to perform a work function.
 26. The control system asrecited in claim 25, wherein the hydraulic cylinder comprises a liftingcylinder (108, 109) for moving a loading arm (106) which is pivotablyconnected to a vehicle frame, the implement (107) being arranged on theloading arm (106).
 27. The control system as recited in claim 25,wherein the hydraulic cylinder comprises a tilting cylinder (1 10, 902)for moving the implement (107), which is pivotably connected to aloading arm (106), which is in turn pivotably connected to a vehicleframe.
 28. The control system as recited in claim 27, wherein thetilting cylinder (110) is adapted so that a load (904) which acts on thetilting cylinder draws the piston rod of the tilting cylinder out viaits weight.
 29. The control system as recited in claim 28, wherein thecontrol system comprises an additional, smaller pump (908), which has adriving connection to the hydraulic machine (204), and wherein thisadditional pump (908) is connected to the piston side (906) of thetilting cylinder and to the tank (216) in order to pump hydraulic fluidto the piston side during a lowering movement.
 30. The control system asrecited in claim 29, wherein the control system comprises a non-returnvalve (914) which is connected between an inlet side (916) and an outletside (918) of the additional pump (908) so that the pump (908) onlypumps hydraulic fluid in a circuit (920) comprising the non-return valve(914) during a lifting movement.
 31. A method for controlling ahydraulic cylinder (108, 109, 110) of a work machine (101 ), whichhydraulic cylinder is adapted to move an implement (107) which issubjected to a load (116), the hydraulic cylinder being controlled by ahydraulic machine (204), said method comprising: detecting that alowering movement of the implement is initiated, and of before thelowering movement takes place driving the hydraulic machine (204) in afirst rotation direction so that a first line (220) which connects thehydraulic machine (204) to said first side (208) of the piston (218) ofthe hydraulic cylinder is pressurized, which side is opposite a secondside (212) on which said load acts.
 32. The method as recited in claim31, further comprising, after the pressurization, allowing the hydraulicmachine (204) to rotate in a second rotation direction, opposite thefirst rotation direction, whereupon the lowering movement can start anda hydraulic flow from the hydraulic cylinder drives the hydraulicmachine (204) in the second rotation direction.
 33. The method asrecited in claim 31, further comprising a controllable means (243) foropening and closing a flow connection between the hydraulic machine(204) and the hydraulic cylinder (108) being arranged on the first line(210), comprising the steps of keeping the controllable means (243)closed for flow in the direction from the hydraulic cylinder to thehydraulic machine (204) and of pressurizing the line (210) between thehydraulic cylinder (204) and the controllable means (243).
 34. Themethod as recited in claim 33, further comprising, after thepressurization, opening the controllable means (243) in order to allowthe hydraulic machine (204) to rotate in a second rotation direction,opposite the first rotation direction, whereupon the lowering movementcan start and a hydraulic flow from the hydraulic cylinder drives thehydraulic machine (204) in the second rotation direction.
 35. The methodas recited in claim 31, further comprising the step of graduallyreducing the pressurization so that a smooth lowering movement isachieved.
 36. A method for controlling a hydraulic cylinder (108, 109,110) of a work machine (101) for the purpose of moving an implement(107) which is connected to the hydraulic cylinder, a hydraulic machine(204) providing the hydraulic cylinder with pressurized hydraulic fluid,said method comprising: detecting a load (116) acting on the implement(107); comparing the size of the detected load with a predetermined loadlevel, and, if the detected load lies below the predetermined loadlevel, bringing the piston-rod side (212) of the hydraulic cylinder intoflow communication with the piston side (208) so that hydraulic fluidcoming from the piston-rod side (212) is brought to the piston side(208) without passing through the hydraulic machine (204).
 37. Themethod as recited in claim 36, the hydraulic machine (204) providing thehydraulic cylinder with pressurized hydraulic fluid from a first port(220), comprising the step of, if the detected load (116) exceeds thepredetermined load level, bringing the piston-rod side (212) of thehydraulic cylinder into flow communication with a second port (222) ofthe hydraulic machine (204) so that hydraulic fluid coming from thepiston-rod side (212) is brought to the second port (220) of thehydraulic machine (204).
 38. A method for regenerating energy when animplement (107) of a work machine (101) is moved during movement of thework machine, at least one hydraulic cylinder (108, 109, 110) beingconnected to the implement for controlling its movements, said methodcomprising: delivering such a pressure to the hydraulic cylinder thatthe implement is brought into a basic position, of, in the event of adisturbance which results in a downward movement of the implement;allowing driving of a hydraulic machine (204) by a hydraulic fluid flowfrom the hydraulic cylinder; and regenerating the energy from thehydraulic machine (204) in an electric machine (202) connected to it ina driving manner.
 39. The method as recited in claim 38, comprising thestep of bringing a first port (220) of the hydraulic machine (204) intoflow communication with a piston side (208) of the hydraulic cylindervia a first line (210) and a second port (222) of the hydraulic machine(204) into flow communication with a piston-rod side (212) of thehydraulic cylinder via a second line (214).
 40. The method as recited inclaim 38, comprising the step of controlling the hydraulic machine (204)so as to deliver such a pressure to the hydraulic cylinder that theimplement is brought into said basic position.
 41. The method as recitedin claim 38, comprising the steps of repeatedly detecting the positionof the implement (107) and of, in the event of a disturbance whichresults in an upward movement of the implement, supplying acorresponding quantity of hydraulic fluid to the hydraulic cylinder and,in the event of a downward movement of the implement, draining acorresponding quantity of hydraulic fluid from the hydraulic cylinder.42. The method as recited in claim 38, comprising the step of theelectric machine (202) driving the hydraulic machine (204) so that thehydraulic fluid is supplied to the hydraulic cylinder for upwardmovement of the implement.
 43. The method as recited in claim 38,comprising the step of continuously controlling the hydraulic cylinderso that the implement is kept within a predetermined range around thebasic position.
 44. A method for springing a movement of an implement(107) of a work machine (101) during movement of the work machine, atleast one hydraulic cylinder (108, 109, 110) being connected to theimplement for controlling its movements, said method comprising:delivering such a pressure to the hydraulic cylinder that the implementis brought into a basic position; and bringing a first port (220) of thehydraulic machine (204) into flow communication with a piston side (208)of the hydraulic cylinder via a first line (210) and a second port (222)of the hydraulic machine (204) into flow communication with a piston-rodside (212) of the hydraulic cylinder via a second line (210), and of, inthe event of a disturbance which results in an upward movement of theimplement, supplying a corresponding quantity of hydraulic fluid to thehydraulic cylinder and, in the event of a downward movement of theimplement, draining a corresponding quantity of hydraulic fluid from thehydraulic cylinder.
 45. The method as recited in claim 44, furthercomprising the steps of, in the event of a disturbance which results ina downward movement of the implement (107), allowing driving of thehydraulic machine (204) by a hydraulic fluid flow from the hydrauliccylinder, and of regenerating the energy from the hydraulic machine(204) in the electric machine (202).
 46. The method as recited in claim44, further comprising the step of the electric machine (202)controlling the hydraulic machine (204) to pump hydraulic fluid to thehydraulic cylinder.
 47. The method as recited in claim 44, furthercomprising the step of controlling the hydraulic machine so as todeliver such a pressure to the hydraulic cylinder that the implement isbrought into said basic position.
 48. The method as recited in claim 44,further comprising the steps of at least one operating parameter beingdetected and of the springing movement being controlled according to aspring characteristic depending on the detected operating parameter. 49.The method as recited in claim 44, further comprising the steps of atleast one operating parameter being detected and of a damping movementbeing controlled depending on the detected operating parameter.
 50. Themethod as recited in claim 48, further comprising the steps of detectinga level of the disturbance force and of controlling the springing and/orthe damping depending on the disturbance force level.
 51. The method asrecited in claim 48, further comprising the steps of detecting theweight of the load and of controlling the springing and/or the dampingdepending on the weight.
 52. The method as recited in claim 48, furthercomprising the steps of detecting the type of implement and ofcontrolling the springing and/or the damping depending on the detectedimplement type.
 53. The method as recited in claim 48, furthercomprising the steps of detecting the type of handling mode and ofcontrolling the springing and/or the damping depending on the detectedhandling mode.
 54. A control system for a work machine (101) comprising:a first subsystem (707, 709) for performing a first work operation and asecond subsystem (731) for performing at least one second workoperation; said first subsystem comprises an electric machine (202), ahydraulic machine (204) and at least one hydraulic cylinder (108, 109,110), the electric machine (202) being connected in a driving manner tothe hydraulic machine (204), the hydraulic machine (204) being connectedto a piston side of the hydraulic cylinder via a first line and apiston-rod side of the hydraulic cylinder via a second line, thehydraulic machine being adapted to be driven by the electric machine andsupply the hydraulic cylinder with pressurized hydraulic fluid from atank in a first operating state and to be driven by a hydraulic fluidflow from the hydraulic cylinder and drive the electric machine in asecond operating state; said second subsystem (731) comprises a driveunit (734) and a hydraulic actuator (732), the drive unit (734)comprising an electric machine (738) and a hydraulic machine (736), theelectric machine being connected in a driving manner to the hydraulicmachine, the hydraulic machine (736) being adapted for flowcommunication with the hydraulic actuator (732), and a means (748, 750,756, 758) being adapted for controlling the movement of the hydraulicactuator (732).
 55. The control system as recited in claim 54, whereinsaid control means (748, 750) is arranged on an inlet side of thehydraulic actuator (732).
 56. The control system as recited in claim 54,wherein said control means (756, 758) is arranged on an outlet side ofthe hydraulic actuator (732).
 57. The control system as recited in claim54, wherein said control means (748, 750, 756, 758) comprises at leastone valve.
 58. The control system as recited in claim 54, wherein thecontrol system comprises a third subsystem (711) for frame-steering thevehicle (101) and said third subsystem (711) comprising a first steeringcylinder (104) and a second steering cylinder (105), which steeringcylinders are adapted for frame-steering the vehicle, a first drive unit(704) and a second drive unit (706), which each comprise an electricmachine (708, 710) and a hydraulic machine (712, 714), each electricmachine being connected in a driving manner to its associated hydraulicmachine, a first (712) of the two hydraulic machines being adapted forflow communication with a piston side (716) of the first steeringcylinder (104) and a piston-rod side (718) of the second steeringcylinder (105), a second (714) of the two hydraulic machines beingadapted for flow communication with a piston side (720) of the secondsteering cylinder and a piston-rod side (722) of the first steeringcylinder.
 59. A method for limiting a pressure which is delivered by ahydraulic machine (204, 705, 712, 714, 736) forming part of a controlsystem, when the hydraulic machine is used as a pump, an electricmachine (202, 703, 708, 710, 738) being connected in a driving manner tothe hydraulic machine (204), said method comprising: detecting anoperating parameter and of generating a corresponding parameter signal;determining a level of said pressure based on the level of the detectedoperating parameter; and comparing the determined pressure level with apredetermined maximum level and of controlling the hydraulic machine sothat a delivered pressure lies below the predetermined maximum level.60. The method as recited in claim 59, further comprising detecting atorque delivered by the electric machine and of determining the level ofsaid pressure based on the detected torque.
 61. The method as recited inclaim 60, further comprising calculating a level of said pressure basedon at least the detected torque and the displacement of the hydraulicmachine.
 62. The method as recited in claim 59, further comprisingdetecting the pressure of the hydraulic fluid in a line downstream ofthe hydraulic machine and of comparing the detected pressure level withthe predetermined maximum level.
 63. The method as recited in claim 59,further comprising: providing the hydraulic machine with an hydraulicactuator (104, 105, 108, 109, 110, 732, 902) with pressurized hydraulicfluid.